A microreactor is a miniaturized reaction system for process engineering and chemical engineering. Such microreactors are provided with fluid channels, reaction chambers, reaction spaces, heating devices, mixing devices or the like, for example, and several such elements may also be combined to form a microreactor system. Such a microreactor may be used, for example, for individual operations such as performing chemical, biochemical and physicochemical reactions, distillation, mixing, separating, etc., or for an entire chain of such operations.
Such a microreactor for chemical, biochemical or physicochemical reactions usually has one or more inlets or feeder lines and one or more outlets or outlet lines for the product(s) (reaction product) formed from the reactants, such that between them there is at least one reaction space or reaction chamber, in which the at least one reactant is converted into the product(s). Catalysts or catalytically active elements are often used with such reactions, specifically accelerating the particular reaction without being converted themselves, by lowering the activation energy.
The catalytic reactions performed in such a reactor include, for example, steam reforming, CO2 reforming, (partial) oxidation, chlorination, fluorination, hydrogenation, dehydrogenation, nitration, CO conversion, reverse reaction of CO conversion, autothermal reforming, incineration, hydrocracking, reduction, partial reduction and hydrodesulfurization.
Microreactors with the largest possible specific surface area are useful in particular for catalytic reactions between fluids (reactions between gases and/or liquids) to achieve a high reaction rate with the lowest possible use of the reactants as materials.
The document DE 103 13 685 A1 discloses a microreactor comprising several individual layers combined to form a stack of layers. The stack of layers here has at least one cover layer and at least one channel layer, such that on one side the channel layer is provided with microstructures, which together with the facing side of the neighboring cover layer, form microchannels. The microstructured layer may be manufactured here from a metal foil, for example, a copper foil, aluminum foil or stainless steel foil, or manufactured from a plastic film, a ceramic film or other materials. The microstructures can be produced by etching, whereby a semi-elliptical cross section of the individual microstructures is obtained by the etching process. Embossing is mentioned in this document as an alternative to etching to produce the microstructures, allowing lower machining costs in comparison with etching. The microchannels formed by a channel layer and a cover layer have a width in the range of 400 μm, a depth of 200 μm and a length of 8 mm to 20 mm. The disadvantage of such a microreactor is that the ratio of available reaction surface area is low in relation to the volume of the microreactor, so the microreactor must be constructed to be comparatively large on the whole in order to provide the desired specific surface area.
Document EP 1 206 316 B1 discloses a chemical reactor for a catalytic reaction having at least one gaseous reactant. The reactor has a porous structure, which is arranged in such a way that the flow path of the reactant runs along a microchannel and in contact with the porous material. A portion of the at least one reactant diffuses in a transverse molecular pattern into the porous structure to which a catalytically active material can be applied, and it reacts to form at least one product that diffuses transversely back into the flow path. The porous material may comprise a catalytically active material, e.g., a catalytic ceramic or a catalytic metal in the form of a foam or felt. Alternatively, the porous material may comprise a porous carrier of a noncatalytic material (e.g., a ceramic foam), whereby a catalytically active material is arranged on this support. The porosity may be designed as a geometrically regular pore structure, e.g., a honeycomb structure or a structure comprising parallel pores, or may be designed as a geometrically coiled or random structure. According to the specifications in the document, the degree of porosity may be approx. 30%, for example, up to approx. 98%, whereby the average pore size is smaller than the smallest dimensions of the microchannels. The pore size is approx. 0.1 μm to approx. 200 μm, thus allowing molecular diffusion.
The documents U.S. Pat. No. 5,674,301 and WO 00/06295 A1 also describe microreactors in which a catalyst is arranged on a carrier having pores or porous structures. The document U.S. Pat. No. 5,674,301 discloses a porous substrate with which a catalyst is provided in the pores for reforming hydrocarbon. Document WO 00/06295 A1 describes a reactor device having a reaction chamber, whereby a porous insert through which essentially all the reactants must pass is provided in the volume of the reaction chamber. The porous insert has an average porosity of less than 1 and has a transport distance of no more than 3 mm. The porous insert here may comprise a powder, a porous monolithic material (e.g., an expanded metal or a ceramic foam), a honeycomb structure, a tube row, a module with microchannels stacked one above the other or a combination of these structures), or fibers. For catalytic reactions, the porous insert may comprise a porous carrier with a catalyst material arranged on it.
The disadvantage of these known microreactors is that the catalytically active material is applied to the porous surface area so that the surface area available for reaction of the reactants with the catalytically active material is greatly limited. Therefore, the large surface area available through the porosity is thus not utilized. Furthermore, the document EP 1 206 316 B1 describes an expanded ceramic material as a substrate comprising in its totality a porous structure. The areas of the porous structure can be determined only by the external dimensions.